Polypropylene and polypropylene copolymers may be formed in a polymerization reactor in the presence of an appropriate catalyst. Propylene monomer is introduced into the reactor, alone or in combination with one or more other comonomers, such as ethylene, to produce the polypropylene homopolymer or copolymer fluff or granules. The propylene polymer is withdrawn from the reactor and may be subjected to appropriate processing steps and then extruded as a thermoplastic mass through an extruder and die mechanism to produce the propylene polymer as a raw material in particulate form, usually as pellets. The propylene polymer pellets are ultimately heated and processed in the formation of the desired end products. Examples of such end products include, but are not necessarily limited to, fibers, webs (both woven and nonwoven), films, pipe, containers, and foamed articles. Other examples of such products made from propylene polymers include component parts of durable goods articles such as automotive interior and exterior components, and household appliance interior and exterior components
One form of reactor suitable for polypropylene homopolymer and copolymer production is a bulk loop reactor. A bulk loop reactor may be formed from one or more interconnected loops having a continuous bore. The catalyst is distributed within the continuous bore by circulating liquid propylene monomer. In this way, propylene polymer polymerization occurs within the continuous bore. Such reactors may also be called slurry loop reactors. Two or more bulk loop reactors may be connected, such as for example in series. Herein, all of these reactors are termed “loop-type” reactors. In this way, the polymerization conditions in each reactor may be the same or different to achieve desired polymer properties. Examples of polymerization conditions that may be varied include temperature, pressure, monomer content, comonomer content, catalyst, co-catalyst, residence time, and, optionally, hydrogen concentration.
Optionally, the propylene in a mixture of reaction medium and catalyst may form a mixture or “paste”, along with any co-catalyst or internal or external electron donor, if used, may be placed into a pre-polymerization reactor, which may be a loop reactor, prior to introduction into the main slurry loop reactor. Propylene polymer granules are formed as propylene polymerization begins upon contact between the catalyst/co-catalyst and the liquid propylene monomer, all of which are circulated within the pre-polymerization loop reactor by a circulation pump.
Polymer particles exiting the bulk loop reactor may be subjected to processing steps as described above or they may be introduced into one or more additional polymerization reactors, such as for example one or more gas phase reactors, for further polymerization with other monomers, such as ethylene, to alter the physical and chemical properties of the propylene polymer resins. Additionally, the physical and chemical properties of the propylene polymer resin may be tailored by the selection of one or more catalyst systems.
Because bulk loop reactors can produce propylene polymers on a substantially continuous basis, and at high out-puts over an extended period of time, such as for example, from between 1 to at least 50 tons of propylene polymer per hour for between 5 days to up to 2 years and beyond, bulk loop reactors offer several advantages over other types of polypropylene reactors, such as stirred pot, stirred bed, and other non-substantially continuous reactors.
Copolymers of polypropylene having low melting points may be obtained by inserting comonomers in the polymer chain during polymerization. The addition of ethylene and/or other comonomers in the growing chains of polypropylene during polymerization gives rise to a propylene copolymer that may be characterized by a lower melting point, a lower flexural modulus, lower rigidity, higher transparency and lower crystallinity than the homopolymers of propylene. The comonomers may generate defects in the polymer chain which impede the growth of thick crystalline structures and reduce the degree of crystallinity of the overall polymer. The comonomers are not evenly distributed in the polymer chains. Among the many comonomers that may be used in the copolymerization process, ethylene and butene have been most common. It has been observed that the melting temperature of the propylene copolymers is reduced by about 6° C. per wt % of inserted ethylene in the copolymer chain or by about 3° C. per wt % of inserted butene. The addition of comonomer in industrial polymerization processes has other effects than just decreasing the melting temperature of the polypropylene; it has both economical and technical effects.
Besides using Ziegler-Natta (ZN) catalysts, new metallocene catalysts show potential to produce this type of material in loop reactors. However, due to stickiness of this soft material, the copolymer fluff grains tend to agglomerate and thus cause the circulation power (amps) to increase and become erratic and thus discontinue the steady production. In fact, severe agglomeration would generate relatively large copolymer accumulations or “rocks”, and even render post-reactor process suspended (e.g. due to the plugging of valves, etc.). The copolymer agglomerations may cause undesirable vibration in the pump motors and the reactor itself. Generally, the copolymer fluff grains may lose bulk density, that is lose mass per unit volume that the fluff grains occupy. This results in circulator problems and decreases the production rate (throughput).
Another situation aggravated by copolymer fluff stickiness concerns devolatizing the fluff grains of the reaction medium. Generally, after the polymerization is ceased by deactivating or “killing” the catalyst, the copolymer fluff granules are heated so that upon entering a flash tank, a substantial portion of the liquid propylene monomer and/or reaction medium accompanying the copolymer fluff granules vaporizes, thus separating from the granules. The gaseous propylene and a portion of polymerization by-products are extracted from the flash tank. A nitrogen purge may be optionally used to remove the last of the propylene or reaction medium. Suitable kill materials may be one of many known protic substances, having an active hydrogen including, but not necessarily limited to, heteroatom-containing materials, e.g. alcohols, glycols, amines, CO2, CO, etc.
It is desirable that the copolymer fluff granules are small, spherical and uniform so that excess propylene and/or reaction medium may be easily removed. If the granules become too big, it is difficult to get all of the propylene and/or reaction medium out of the fluff. Pellets containing excess diluent or reaction medium may be explosive, and granules having propylene may catch fire.
Other difficulties may occur when the copolymer granules exit the flash tank if they contain appreciable amounts of diluent or reaction medium. The conveying equipment moving the fluff granules may not function properly and gravity feeds will not work properly if the fluff is sticky.
Thus, alleviating the stickiness and agglomeration tendency of the copolymer fluff inside the loop media but retaining the same melting point in the final pellets would be helpful to resolving this technical challenge. The stickiness appears to be related to the amount of the amorphous part, which is associated to the comonomer (e.g. ethylene) content for a specific catalyst system under steady polymerization conditions. Higher amorphous content dispersed among the crystalline domains is believed to make the fluff stickier, without wishing to be limited to any particular theory.
Other techniques and methods have been used to attempt to improve fluff particle morphology. For instance, changes in reaction temperature, varying the copolymer bulk density, using a different catalyst or external electron donor used as separate or combined improvements may help improve fluff morphology and circulation. Prepolymerization has also been used to give better fluff morphology, and the comonomer addition rate may also affect particle morphology.
It is a continuing goal of the industry to produce RCP having improved properties, such as a melting point on the order of about 100 to about 170° C. and reduced tendency of the RCP fluff to agglomerate or stick together during manufacturing and post-reactor finishing.